1. Field of the Invention
The present invention relates to an automatic convergence correcting circuit for controlling an electron beam of a cathode ray tube and more particularly to such a circuit that is adapted to effect automatic convergence correction by the use of index patterns.
2. Description of the Prior Art
There has been proposed a cathode ray tube including a circuit that is adapted to effect automatic convergence correction by the use of index patterns provided on the back of a shadow mask (U.S. Pat. Nos. 4,617,495 and 4,456,853).
That is, in a cathode ray tube 1, as shown in FIG. 2, a phosphor material is applied to the surface of a shadow mask 2 on the side thereof that faces towards the electron gun, whereby position detecting patterns 3 (hereinafter called index patterns) respectively representative of different positions on the display screen are formed. Thus, when an electron beam 4 makes a raster scan of a display screen 5 along horizontal scanning lines through the shadow mask, the electron beam 4 illuminates the index patterns 3 so that light is emitted therefrom, and this light is converted into a detection signal S11 by, for example, a photodetector 11 provided at the cone portion of the cathode ray tube. A convergence circuit is controlled with the thus detected signal, and the deflected position of the electron beam 4 is thereby automatically adjusted.
The index pattern typically has the form of a so-called lambda pattern. That is, it is formed, as FIG. 6(A) shows, of a first belt-like or strip pattern section 3A extending in the vertical direction and a second belt-like or strip pattern section 3B extending in an oblique direction, for example at an angle of 30.degree. with the horizontal direction, from a position a predetermined distance apart from the first pattern section and in confronting relation thereto. Such lambda patterns are distributed all over the display screen 5 and form a matrix with predetermined spacings in the horizontal and vertical directions.
Thus, when the electron beam 4 makes a horizontal scan from one reference position PSO to the other reference position PS3 along a scanning line SCNO at a level virtually in the middle of the height in the vertical direction of the first and second pattern sections 3A and 3B (FIG. 6(A)), abruptly rising pulse signals PA and PB are generated in succession by the photodetector 11 at the times t.sub.a and t.sub.b when the electron beam 4 arrives at the positions to start illuminating the pattern sections 3A and 3B (FIG. 6(B)).
Since, in FIG. 6A, the first pattern section 3A extends in the vertical direction, the time at which the pulse signal PA rises indicates the position of the index pattern in the horizontal direction on the display screen. On the other hand, since the second pattern section 3B extends in an oblique direction, the period of time that elapses from the time at which the pulse signal PA rises to the time at which the pulse signal PB rises represents the position in the vertical direction of the index pattern 3.
At the time t.sub.0, when a red electron beam, for example, passes the illumination starting point, an integral controlling signal S2 (FIG. 6(C)) for a ramp signal generator is lowered to a logical level "L" to start an integrating operation, whereby a ramp signal S3 (FIG. 6(D)) is generated. Thereafter, at the rising time t.sub.1 of the pulse signal PA (FIG. 6(B)) to be detected as the electron beam passes the pattern section 3A, the integral controlling signal S2 is reset to a logical level "H" and thereby the integration is stopped. The value of the ramp signal S3 at this time is stored as the detected value of the position of the index pattern in the horizontal direction.
Detected values S3 of the positions in the horizontal direction for the green and blue electron beams are obtained similarly.
The thus obtained detected values S3 of the positions in the horizontal direction are compared with a reference value of the position in the horizontal direction. Any deviation that such a comparison reveals indicates that an error is present in the horizontal convergence of the electron beam, and hence a horizontal convergence error signal is supplied to a convergence and deflection circuit so that the error is corrected.
Then, at the rising time t.sub.1 of the pulse signal PA corresponding to the red electron beam, for example, which occurs when the electron beam reaches the pattern section 3A, an integral controlling signal S4 (FIG. 6(E)) is lowered to a logical level "L" to start an integrating operation in the ramp signal generator, whereby a ramp signal S5 FIG. 6(F)) is generated. Thereafter, at the rising time t.sub.2 of the pulse signal PB, which occurs when the electron beam reaches the second pattern section 3B, the integral controlling signal S4 is reset to a logical level "H", so that the integrating operation is stopped. The value of the ramp signal S5 at this time is stored as the detected value corresponding to the position of the index pattern in the vertical direction.
Detected values S5 of the positions in the vertical direction for the green and blue electron beams are obtained similarly.
The thus obtained detected values of the positions in the vertical direction are compared with a reference value of the position in the vertical direction, and if there is any deviation, it indicates that an error is present in the vertical convergence of the electron beam, and hence a vertical convergence error signal is supplied to the convergence and deflection circuit so that the error is corrected.
Thus, deflection waveform signals capable of convergence correction are obtained based on the horizontal and vertical error signals, and convergence correction coils are driven in accordance with these signals so that the convergence is corrected.
However, when an attempt is made to adjust the convergence automatically through the method described above, there has been a problem resulting from the fact that the S/N (signal-to-noise) ratio of the signal S1 detected by the photodetector is deteriorated because the light emitted from the phosphor material forming the index pattern is weak and the signal is susceptible to electrical noise within the cathode ray tube. Hence, the rising times t.sub.1 and t.sub.2 of the pulse signals PA and PB obtained from the detected signal S1 are liable to deviate to a certain degree from the times t.sub.a and t.sub.b at which the electron beam actually reaches the pattern sections 3A and 3B. Therefore, it has been difficult to carry out the automatic convergence adjustment with high accuracy when it is performed based on the times t.sub.1 and t.sub.2.
In practice, in order to obtain the integral controlling signals S2 and S4 from the detected signal S1, a method is used wherein the detected signal S1 is sliced at predetermined slice levels, and the integral controlling signals S2 and S4 are raised from a logical level "L" to a logical level "H" at the rising times of the sliced output. However, if a noise whose signal level is close to the slice level is mixed in with the detected signal S1, then a deviation of the rising time of the sliced output will occur.
To solve that problem, it is possible to perform raster scanning on the entire scanning plane a number of times and perform the integration for each index pattern a number of times, so that the plurality of integrated results are averaged and the portion of the noise component included in the signal is thereby reduced. But that approach gives rise to the problem that the processing time is increased by the need to perform a plurality of raster scans on the scanning plane.